The reaction of tantalum with sulphur vapour and selenium vapour

The kinetics of reaction of coupons of annealed tantalum with sulphur vapour and selenium vapour were studied by a thermogravimetric method over the temperature range 485-5500C. Total sulphur pressures ranged from 0.5 to 40 kPa and total selenium pressures varied from 0.01 to 4 kPa. For the reaction with sulphur vapour, slow linear kinetics were observed under all conditions that yielded a TaS2-type compound. The apparent activation energy calculated from the linear rate constants was 202 kJ mol-1. The rate increased as Psub(S2)sup(0.25) where Psub(S2) is the partial pressure of S2. It is postulated that the sulphurization process is chemically controlled at the surface of the TaS2 scale. For the reaction with selenium vapour, parabolic kinetics were observed under all conditions that yielded a TaSe2-type scale. The apparent activation energy calculated from the parabolic rate constants was 137 kJ mol-1. The parabolic rate constant increased as Psub(Se2)sup(0.51) where Psub(Se2) is the partial pressure of Se2, and this leads to the conclusion that the rate was controlled by the diffusion of tantalum vacancies through the constantly thickening scale. (Auth.)